The present invention relates to automotive braking systems. In particular, the present invention as disclosed herein relates to an automotive braking system that utilizes an eddy current brake and a control methodology therefor.
Electromagnetic retarders or eddy current brakes often have been used to assist in vehicle deceleration, particularly in commercial trucks. In typical prior art eddy current retarders, a brake rotor rotates at the speed of the prime mover until a field coil is energized. Rotation of the rotor is slowed by controlling the current in the field coil. These types of retarders have not found their place in passenger vehicles primarily because they exhibit a lower torque density compared to other friction braking devices. With recent advances in materials and precision air gap retention, it is now possible to obtain significantly higher torque density for eddy current retarders that are configured for improved cooling of the retarder components.
The response characteristics of the new generation of eddy current retarders are relatively rapid compared to other prior art configurations. It is typically desired that the torque response behavior of these devices matches or exceeds the hydraulic friction brake response times. With a conventional PID controller, it is highly unlikely to obtain an improved response behavior without setting the gains too high. High gain settings are undesired because these settings may lead to instability.
Some prior art references address the control strategies for an electromagnetic retarder. U.S. Pat. No. 5,743,599 relates to a control strategy for electromagnetic retarders. The proposed controller includes an open loop that controls the supply current from a current source (battery) to a number of discretely arranged retarder coils. The control system includes a number of switch circuits for energizing or de-energizing the retarder coils.
U.S. Pat. No. 5,187,433 discloses a device for measuring or adjusting braking torques generated by electromagnetic retarders. The device estimates retarder torque via predetermined look-up tables of torque vs. peak voltage between two poles at various rotor speeds.
Simeu and Georges, in 1996, proposed an eddy current brake model as a function of excitation current and rotor speed. However, the disclosed model is more specific to a particular class of eddy current machines. Due to the design of a new generation of eddy current retarders with two to three fold increase in torque density compared to currently available commercial eddy current machines, a more generic control model is desired with the ability to control a range of retarders having various torque density characteristics.
In order to alleviate one or more shortcomings of the prior art, a control method and system are provided herein. In accordance with the present invention, a sliding-mode controller is designed for an eddy current braking system in a vehicle deceleration application.
In one aspect of the current invention, a method for controlling an eddy current braking system in a motor vehicle is provided. The motor vehicle includes a prime mover linked to the eddy current braking system to provide torque thereto, and the eddy current braking system has a retarder assembly including at least a rotor and a stator. The method includes the steps of detecting a feedback current from the retarder assembly, detecting a rotor speed of the rotor, providing a signal indicative of a desired retarding torque, determining a command current for the retarder as a function of the feedback current, rotor speed and desired retarding torque using a closed-loop sliding-mode control algorithm, and providing the command current to the retarder to control application of torque to the prime mover.
In another aspect of the invention, an eddy current braking system for a motor vehicle having a prime mover turning a shaft is described. The system comprises an eddy current brake including a rotor, at least one sensor operably connected to the rotor to detect a rotational speed of the rotor, at least one sensor operably connected to the stator to determine a feedback current of the brake, a computer in communication with the sensors, a torque selector in communication with the computer for selecting a desired retarding torque, and a memory accessible to the computer. The memory stores an algorithm for the computer to determine a command current Icmd as a function of the feedback current, rotor speed and desired retarding torque.
In yet another aspect of the invention, a method for controlling an eddy current braking system in a motor vehicle is provided. The method includes the steps of providing rotor speed and feedback current information to a computer from the eddy current brake, verifying that the rotor speed is above a minimum value, calculating a command current as a function of the rotor speed, feedback current, and desired torque, converting the command current into a pulse-width modulated signal, and providing the signal to the eddy current brake stator.
Simulation results show that the controller herein exhibits clear advantages over the more conventional PID controller. The algorithm and associated implementations disclosed herein exhibit improved robustness and computational efficiency in the controller. The closed-loop control system alleviates the need for an inefficient look-up table which in the past was required to calculate the current command for the brake.